How much of the change from 2016 was due to different people voting vs. the same people changing their vote choice?

A colleague writes:

Whenever I think of appropriate democratic strategies for 2020 I am drawn to ask how a candidate can get voters from Trump.

But a colleague frequently corrects my thinking by saying Karl Rove discovered that you want to rile up your base to get them to turn out and appealing to the median voter is nonsense. He is not the only one who says so.

Is there any evidence you have seen on the Rove hypothesis? What is the best evidence I wonder? Strikes me as a hard thing to study but terribly important.

I guess the first question I would have is how many people vote sometimes (say 30-80% of the time) vs. how many vote sometimes democratic (30-80%) of time. What is the size of the two pools? Harder to get at of course is the elasticity or ease of changing those numbers. But if you assumed they were equally easy to shift, then the size of the pools would be determinative.

Or if the size was comparable and your prior was that shifting a political opinion is very hard but getting someone to decide to vote on a given occasion vs. stay home (if they sometimes vote) is less hard, then maybe with equally sized pools, you know the answer.

Before giving my answer (or, more precisely, Yair’s answer), I’ll just point out that the same question could be asked from the Republican side. Given that the Democrats are doing various things to try to win more votes in 2020, the Republicans can hardly just stand still in this new election and expect to squeak by again. So, although my colleague poses the question from a particular partisan perspective, it applies from either direction.

OK, now to the answer. It turns out I already posted on the topic in May, reporting a detailed analysis from Yair Ghitza, who asked:

How much of the change from 2016 was due to different people voting vs. the same people changing their vote choice?

and who concluded:

Two things happened between 2016 and 2018. First, there was a massive turnout boost that favored Democrats, at least compared to past midterms. . . . But if turnout was the only factor, then Democrats would not have seen nearly the gains that they ended up seeing. Changing vote choice accounted for a +4.5% margin change, out of the +5.0% margin change that was seen overall — a big piece of Democratic victory was due to 2016 Trump voters turning around and voting for Democrats in 2018.

And lots of data-rich detail in between.

I’m reposting because it seems that at least one potential reader didn’t see this when it came up the first time.